Unique electronic properties of silicon carbide (SiC) make it a very desirable material for state-of-the-art semiconductor devices that can operate at high frequencies, high voltages and current densities, and in harsh conditions. In many such devices, silicon carbide is utilized as a substrate on which the semiconductor device structure is formed using epitaxy, photolithography and metallization. Depending on the device design, the substrate must possess specified electronic parameters, such as conductivity type and resistivity. While devices operating at high and microwave frequencies (RF devices) require semi-insulating (SI) substrates with very high resistivity, for other devices, such as high power switching devices, low-resistivity n-type and p-type substrates are needed.
Presently, SiC single crystals are grown on the industrial scale by a sublimation technique called Physical Vapor Transport (PVT). A schematic diagram of a typical prior art PVT arrangement is shown in FIG. 1. In PVT, polycrystalline grains of silicon carbide (SiC source) 1 are loaded on the bottom of a growth container 2 and a SiC seed crystal 4 is attached to a top of growth container 2. Desirably, growth container 2 is made of a material, such as graphite, that is not reactive with SiC or any dopant (discussed hereinafter) added thereto. The loaded growth container 2 is evacuated, filled with inert gas to a certain, desired pressure and heated via at least one heating element 3 (e.g., an RF coil) to a growth temperature, e.g., between 1900° C. and 2400° C. Growth container 2 is heated in such a fashion that a vertical temperature gradient is created making the SiC source 1 temperature higher than that of the SiC seed 4. At high temperatures, silicon carbide of the SiC source 1 sublimes releasing a spectrum of volatile molecular species to the vapor phase. The most abundant of these gaseous species are Si, Si2C and SiC2. Driven by the temperature gradient, they are transported to the SiC seed 4 and condense on it causing growth of a SiC single crystal 5 on the SiC seed 4. Prior art patents in this area include, for example, U.S. Pat. Nos. 6,805,745; 5,683,507; 5,611,955; 5,667,587; 5,746,827; and Re. 34,861, which are all incorporated herein by reference.
Those skilled in the art of semiconductor materials know that production of SiC substrates with desirable electronic properties is impossible without purposeful introduction of certain impurities in a process known as doping. In silicon carbide, the chemical bonds are so exceptionally strong and solid-state diffusion of impurities is so slow that doping in the bulk can be accomplished only at the stage of crystal growth, when the doping element (dopant) incorporates directly into the lattice of the growing SiC crystal 5.
As a particular example of SiC doping during growth, n-type SiC crystals are produced by adding small amounts of gaseous nitrogen (N2) to growth container 2 atmosphere. Nitrogen-doped SiC single crystals with very uniform electrical properties can be readily grown by maintaining appropriate partial pressure of N2 during growth.
With the exception of the nitrogen-doped crystals, attaining uniform electrical properties in other types of SiC crystals, including semi-insulating, p-type and phosphorus doped n-type crystals, is much more difficult because the doping compounds are not gaseous but solid. Vanadium is one particularly important dopant, which is used to produce a high-resistivity semi-insulating SiC crystal. Aluminum is another important dopant used for the growth of conductive crystals of p-type. Other solid dopants include boron, phosphorus, heavy metals and rare earth elements.
Prior art doping of SiC crystals using a solid dopant is carried out by admixing small amounts of impurity directly to the SiC source 1. For instance, vanadium can be introduced in the form of elemental vanadium, vanadium carbide or vanadium silicide. Aluminum can be introduced in the elemental form, aluminum carbide or aluminum silicide. Other suitable solid dopants, such as boron or phosphorus, can be similarly introduced as elements, carbides or silicides. The doping compound can be in the physical form of powder, pieces or chips.
During SiC crystal 5 sublimation growth, multi-step chemical reactions take place between the SiC source 1 and the dopant admixed directly in the SiC source. These reactions proceed through several stages and lead to the formation of multiple intermediary compounds. In the case of vanadium doping, thermodynamic analysis shows that the product of reaction between SiC and vanadium dopant (whether elemental, carbide or silicide) depends on the stoichiometry of SiC. That is, when the SiC source 1 is Si-rich and its composition corresponds to the two-phase equilibrium between SiC and Si, formation of vanadium silicide (VSi2) is likely. When the SiC source is C-rich and its composition corresponds to the two-phase equilibrium between SiC and C, formation of vanadium carbide (VCx) is likely.
It is known that freshly synthesized SiC source 1 is, typically, Si-rich. Due to the incongruent character of SiC sublimation, the initially silicon-rich SiC source 1 gradually becomes carbon-rich. This change in the stoichiometry of the SiC source 1 during sublimation growth causes the following sequence of reactions:
During initial stages of growth, when the SiC source 1 is Si-rich, reaction between vanadium dopant and SiC yields vanadium silicide VSi2.
As the growth progresses and the SiC source 1 becomes more carbon-rich, vanadium silicide converts to intermediate carbo-silicide VCxSiy.
During final stages of growth, when the SiC source 1 is carbon-rich, vanadium carbo-silicide converts into vanadium carbide VCx.
Accordingly, the partial pressure of vanadium-bearing species in the vapor phase decreases from high at the beginning of growth to low at the end. The change in the vanadium partial pressure results in the characteristic concentration profile with too much vanadium in the first-to-grow portions of the SiC crystal 5 boule and too little in the last-to-grow portions. For this reason, electrical properties of SiC crystals grown using the doping technique of prior art are spatially nonuniform and the yield of high electronic quality substrates is low.
The above case of vanadium doping was given for the purpose of example only. Similar problems exist in the cases when other solid dopants are added to the SiC source 1 directly, including, but not limited to aluminum, boron and phosphorus.